.\" Copyright (c) 2012 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH MALLOC_USABLE_SIZE 3  2015-03-02 "GNU" "Linux Programmer's Manual"
.SH NAME
malloc_usable_size \- obtain size of block of memory allocated from heap
.SH SYNOPSIS
.B #include <malloc.h>

.BI "size_t malloc_usable_size (void *" ptr );
.SH DESCRIPTION
The
.BR malloc_usable_size ()
function returns the number of usable bytes in the block pointed to by
.IR ptr ,
a pointer to a block of memory allocated by
.BR malloc (3)
or a related function.
.SH RETURN VALUE
.BR malloc_usable_size ()
returns the number of usable bytes in
the block of allocated memory pointed to by
.IR ptr .
If
.I ptr
is NULL, 0 is returned.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw20 lb lb
l l l.
Interface	Attribute	Value
T{
.BR malloc_usable_size ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
This function is a GNU extension.
.SH NOTES
The value returned by
.BR malloc_usable_size ()
may be greater than the requested size of the allocation because
of alignment and minimum size constraints.
Although the excess bytes can be overwritten by the application
without ill effects,
this is not good programming practice:
the number of excess bytes in an allocation depends on
the underlying implementation.

The main use of this function is for debugging and introspection.
.SH SEE ALSO
.BR malloc (3)
